Professional knowledge for science teaching develops over time and interplays with professional experiences in field. In the present study, we explore student teachers' reflective conversation upon teaching experiences, with pedagogical content knowledge, PCK, as an analytical lens. The empirical data is based on nine meetings, with groups of 3-6 student teachers with an academic degree, at three different periods during their one-year short-track teacher education program. The findings show how student teachers focus on the PCK component "instructional strategies" in their discussion. A difference between the different sets of meetings is the increased presence of discussions regarding assessment of student learning. The findings also elicit different ways of relating components of PCK in varying contexts. The shift over time from a focus on teachers' instructional strategies to also including students' understanding indicates a development toward becoming a teacher. Even though a structured discussion with theoretically grounded didactic questions is established, it is challenging to deepen the discussion when the student teachers' varying teaching experiences are present at the same time. Nevertheless, the study shows the possibilities of structured group discussions about field experiences in a collegial setting in a short-track teacher education program, regarding student teachers' development as "becoming teachers."